(i) Field of the Invention
The present invention relates to the area of protection of the environment. It relates to the purification of industrial gas prior to their release to the atmosphere. More specifically, it relates to the treatment of effluent tail gases coming from nitric acid factories that typically contain, besides oxygen at the rate of 1.5 to 5%, from 0.5% to 5% water, from 50 to 6000 ppmv of nitric oxides and from 500 to 1500 ppmv of N2O, wherein the temperature is generally less than 400° C.
(ii) Description of Related Art
The nitric oxides NOx, which include NO and NO2, are polluting gases, principally by reason of their corrosive action. They are for the most part the cause of acid rain. In the synthesis of nitric acid, they are the gases that are produced by oxidation of ammonia by oxygen over platinum at high temperature. They are retained in absorption towers by being transformed there to nitric acid. They escape in the tail gases, where it is known to eliminate them to a suitable level by catalytic ammonia reduction: it is the process called SCR (denoting selective catalytic reduction), which is implemented thanks to zeolitic catalysts. European patent (EP-B-0393917, Engelhard Corporation) reveals that the beta zeolite associated with iron as promoter, is an effective catalyst for the selective reduction of NOx by ammonia; it makes no reference to nitrous oxide
Nitrogen protoxide or nitrous oxide, of formula N2O is likewise produced at the time of the oxidation of ammonia by air oxygen at high temperature, and it also is produced during the SCR destruction of nitric oxides. For a long time, one hardly worried about eliminating it before release to the atmosphere until an awareness developed that it was a gas having a strong greenhouse effect. Japanese patent JP 08057262 (Babcock Hitachi) recommends the use of beta-iron for bringing down nitrous oxide by reduction with ammonia. The inventor does not refer to the NOx oxides.
The use of a series of catalytic pots in the same installation in order to successively reduce the content of NOx and N2O gases is a solution of little satisfaction industrially. There has therefore been sought a unique catalyst which would simultaneously carry out the destruction of NOx and N2O by ammonia and, under the same operating conditions, in particular at a temperature less than 400° C., which besides has a hydrothermal stability sufficient at 600° C. to resist temperature periods to which it can be subjected under certain circumstances of its use. An SCR catalyst which in a single operation would permit lowering at the same time of all of the nitrogen oxides and, the protoxide such as the acid oxides, would represent a highly appreciated technical advancement. The idea therefore came to the inventors to inquire whether beta-iron would not be the ideal catalyst for such a treatment, despite the well founded prejudice according to which the ammonia SCR catalysts have only the tendency to manufacture nitrous oxide.